The present invention relates to the field of geophysics. More particularly, the present invention pertains to an antenna suitable to measure the electrical characteristics of geologic formations.
The resistivity of geological formations is routinely measured using wire line induction logging tools. The standard configuration is that of a multiturn loop of wire (solenoid), called the transistor, usually wrapped on a core of high permeability material (mu-metal or ferrite), oriented with the axis of solenoid parallel to the axis of the borehole (the z axis). If a current I is driven in the windings of the solenoid, a magnetic field is proposed whose field strength is roughly that of a magnetic dipole of moment Mz where the moment is the product of the number of turns (N), the current (I), the cross sectional area of the solenoid (A), and the effective permeability (xcexc) of the magnetic material filling the core of cross sectional A. The effective xcexc depends on the true xcexc of the material and in a complex way on the length to diameter ratio of the core of the solenoid. Long thin cores typically have the highest effective xcexc.
The same solenoid may be used as a detector or receiver of a time varying magnetic field. Now the changing magnetic field along the axis of the solenoid produces an emf in the windings which is proportional to the time rate of change of the magnetic induction, B, the number of turns N, the cross sectional A, and again the effective xcexc (which in this case increases the flux threading the windings).
Most current induction tools use such transmitters and receivers spaced apart within a borehole. The principles of operation are well known. The transmitter is driven with an alternating current thus producing an alternating magnetic field in the formation proximate to the transmitter. This magnetic field produces an emf in the formation by virtue of Faraday""s law. The emf drives a current which is inversely proportional to the resistivity of the formation. Finally this current produces a magnetic field, called the secondary field that is also inversely proportional to the resistivity. Both the primary and secondary magnetic fields are detected by the receiver. Thus, measurements of the secondary magnetic field at a receiver are dependent on the resistivity of the formation.
In a uniformly horizontally layered medium the induced currents from such a vertical solenoid lie in the horizontal plane and are sensitive to the resistivity parallel to the bedding planes (the longitudinal resistivity, pl). However most geologic formations of interest are an inhomogeneous medium.
What is needed, therefore, is an antenna capable of detecting magnetic fields in an inhomogeneous medium while having physical dimensions suitable for use in a slim borehole tool.
An antenna, and method for forming the same, is provided that is suitable for receiving or transmitting three (3) components of low frequency magnetic fields while having physical dimensions that can be easily housed inside a slim borehole tool. The antenna includes a permeable core and a plurality of sets of windings of conductive wire surrounding said permeable core, with each set of said plurality of sets having a cross-sectional area associated therewith, with the cross-sectional area of one of said plurality of sets orientated transversely to the cross-sectional areas of the remaining sets. Typically, the cross-sectional area of each of the plurality of cross-sectional areas extend orthogonally to the cross-sectional areas of the remaining cross-sectional areas of the plurality of cross-sectional areas.
The core may be formed either from ferrite, mu-metal or any other suitable material and may have a cylindrical shape or a cubical shape. Typically, the core has a shape that maximizes that amount of material that may be placed in a unit volume. One example of the core includes a plurality of permeable cylindrical rods, while another example includes a plurality of permeable strips. Additionally, an electrostatic shield, consisting of two overlapping helical strips of conductive tape, may be wrapped around the conductive wires. In this manner, electric charges from the windings may be reduced by connecting the conductive tape to the ground.